Laser gas analyzer

ABSTRACT

A laser gas analyzer includes a wavelength-variable laser having a wide wavelength-variable width, a light-split module configured to split an output light of the wavelength-variable laser into a measurement light and a reference light, a first gas cell into which gases to be measured are introduced, and the measurement light is made to be incident, and a data processor configured to obtain an absorption spectrum of each of the gases to be measured based on a reference signal related to the reference light and an absorption signal related to an output light of the first gas cell, and to obtain concentrations of the respective gases to be measured.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application is a continuation of U.S. application Ser. No.13/687,456 filed on Nov. 28, 2012, which is based on and claims thebenefit of priority of Japanese Patent Application No. 2011-258910,filed on Nov. 28, 2011, the contents of which are incorporated herein byreference in their entirety.

BACKGROUND

1. Technical Field

The present disclosure relates to a laser gas analyzer that canefficiently measure a hydrocarbon multicomponent mixed gas in whichmultiple hydrocarbon components are mixed.

2. Related Art

A laser gas analyzer using tunable diode laser absorption spectroscopy(TDLAS) method has an advantage of a capability of measuring theconcentration of a measurement subject component such as ahigh-temperature or corrosive gas in a highly component-selective,non-contact, and fast manner on a real time basis without interferenceby other components simply by irradiating light to a measurement subjectfrom a wavelength-variable semiconductor laser.

FIG. 19 is a block diagram showing an example of a laser gas analyzer ofthe related art using the TDLAS method, and the laser gas analyzerincludes a light source unit including a semiconductor laser thatirradiates measurement laser light toward a measurement gas atmosphere,a light-receiving element that detects the measurement laser light whichhas penetrated through a measurement space of the measurement gasatmosphere, and a detecting unit including a computation processingmodule that processes output signals of the light-receiving element.

The laser gas analyzer shown in FIG. 19 measures the intrinsic molecularoptical absorption spectrums caused by the vibrational and rotationalenergy transitions of measurement subject component molecules present inan infrared to near infrared range using a semiconductor laser having anextremely narrow oscillation wavelength spectrum line width. Themolecular absorption spectrums of most molecules such as O₂, NH₃, H₂O,CO, and CO₂ are present in an infrared to near infrared range, and theconcentration of a subject component can be computed by measuring theoptical absorption amount (absorbance) at a specific wavelength.

In FIG. 19, a semiconductor laser 11 provided in a light source unit 10irradiates a measurement laser light to the atmosphere of a measurementgas 20. Since the laser light that the semiconductor laser 11 outputshas an extremely narrow oscillation wavelength spectrum line width, andcan change the oscillation wavelength by changing the laser temperatureor driving current, only one of the respective absorption peaks of theabsorption spectrum can be measured.

Therefore, an absorption peak not influenced by an interfering gas canbe selected, the wavelength selectivity is high, and there is noinfluence of other interfering components, and therefore a process gascan be directly measured without removing the interfering gas in a stepprior to measurement.

An accurate spectrum that does not overlap with the interferingcomponents can be measured by scanning the oscillation wavelength of thesemiconductor laser 11 in the vicinity of one absorption line of themeasurement component, but the spectrum shape changes due to abroadening phenomenon of the spectrum which is caused by the measurementgas temperature, the measurement gas pressure, coexisting gascomponents, and the like. Therefore, in an actual process measurementaccompanied by environment changes, correction for the changes isrequired.

Therefore, the apparatus of FIG. 19 uses a spectrum area method in whichthe spectrum area is obtained by scanning the oscillation wavelength ofthe semiconductor laser 11 and measuring the absorption spectrum, andthe spectrum area is converted into the component concentration.

Other laser gas analyzers use a peak height method in which ameasurement component is obtained from the peak height of an absorptionspectrum, or a 2f method in which a wavelength scanning signal ismodulated and the concentration of a measurement component is obtainedfrom the peak to peak (P—P) value of the doubled frequency-modulatedwave form of the frequency. However, theses methods are liable to besignificantly influenced by changes in temperature, pressure, coexistinggas components, and the like.

In contrast, in principle, the spectrum area is not influenced bychanges due to the difference of coexisting gas components (the spectrumarea is almost constant regardless of the coexisting gas components),and the spectrum area, in principle, also linearly changes with respectto a pressure change.

In the peak height method or the 2f method, the above three factorscausing change (temperature, pressure, and coexisting gas components)all have a non-linear influence, and, in a case in which the factorscausing change coexist, correction is difficult. However, according tothe spectrum area method, linear correction with respect to a gaspressure change and nonlinear correction with respect to a gastemperature change are possible, and accurate correction can berealized.

The measurement laser light that has penetrated through the atmosphereof the measurement gas 20 is received by the light-receiving element 31provided in a detecting unit 30, and is converted into an electricalsignal.

The output signals of the light-receiving element 31 are adjusted to anappropriate amplitude level through a gain-variable amplifier 32,inputted to an A/D convertor 33, and converted into digital signals.

The output data of the A/D convertor 33 are subjected to repetition of apredetermined number (for example, several hundreds to several thousandsof times) of integration between an integrator 34 and a memory 35 andstorage in the memory 35 in synchronization with scanning of thewavelength of the semiconductor laser 11 so as to remove noise includedin measurement signals, and the data are flattened, and then, inputtedto a CPU 36.

The CPU 36 performs a computation processing such as the concentrationanalysis of the measurement gas based on the measurement signals fromwhich noise is removed, and performs the gain adjustment of theamplifier 32 in a case in which the amplitude level of the output signalof the light-receiving element 31 is not appropriate as the input levelof the A/D convertor 33.

Non Patent Document 1 describes the measurement principle, features, andspecific measurement examples of laser gas analysis to whichwavelength-variable semiconductor laser spectroscopy is applied.

RELATED ART DOCUMENT Non Patent Document

-   [Non Patent Document 1] Kazuto Tamura, and three other authors,    “Laser Gas Analyzer TDLS200 and Its Application to Industrial    Processes,” Yokogawa Technical Report, Yokogawa Electric    Corporation, 2010, Vol. 53, No. 2 (2010), p. 51 to 54

However, in a laser gas analyzer having the configuration shown in FIG.19, since the wavelength-variable range of the semiconductor laser 11 isnarrow, only measurement of a single component is possible.

For example, in a case in which a hydrocarbon multicomponent mixed gasis measured, since the structure is complex, and a number of absorptionlines overlap in hydrocarbons other than CH₄, broad absorption ispresent in the base as well as sharp absorption lines, and thereforethere is no wavelength having no absorption. Therefore, changes in thebase lines due to a change in the transmissivity of gas cells themselvesand the like cannot be corrected.

In addition, in a case in which a hydrocarbon multicomponent mixed gasis measured, a method in which the concentrations (gas partialpressures) of the respective hydrocarbons are obtained from anabsorption spectrum in which there is broad overlapping absorption ofhydrocarbon other than CH₄ becomes necessary.

As such a method, a statistical method (chemometrics) was known in thepast, but a separate calibration curve needs to be obtained for eachapplication, and therefore excessive engineering man-hours are caused.

On the other hand, for measurement of a single component, an area methodwhich does not rely on the statistical method as described in Non PatentDocument 1 is used.

In order to measure the multicomponent mixed gas in which there is broadoverlapping absorption using the area method that does not depend on thestatistical method, it is necessary to separate the absorption spectrumsof the respective components from the overlapped absorption spectrum,and the area method has not yet been put into practical use inanalyzers.

SUMMARY

One or more exemplary embodiments of the present invention provide alaser gas analyzer in which a wavelength-variable laser having a widewavelength-variable width as a laser light source is used, and theconcentrations of the respective components included in themulticomponent mixed gas can be measured relatively easily using thearea method that does not depend on the statistical method.

A laser gas analyzer according to an exemplary embodiment of theinvention comprises:

a wavelength-variable laser having a wide wavelength-variable width;

a light-split module configured to split an output light of thewavelength-variable laser into a measurement light and a referencelight;

a first gas cell into which gases to be measured are introduced, and themeasurement light is made to be incident; and

a data processor configured to obtain an absorption spectrum of each ofthe gases to be measured based on a reference signal related to thereference light and an absorption signal related to an output light ofthe first gas cell, and to obtain concentrations of the respective gasesto be measured.

The data processor may include;

an absorption line wavelength data storage configured to storeabsorption line wavelength data of the gases to be measured;

a wavelength calibrator into which the reference signal and theabsorption signal are inputted and which is connected to the absorptionline wavelength data storage, configured to calibrate wavelength of theabsorption spectrum based on the absorption line wavelength data, thereference signal, and the absorption signal, and to obtain an absorptionspectrum of an absorbance;

an absorption spectrum data storage configured to stores absorptionspectrum data of the gases to be measured;

a spectrum separator into which the absorption spectrum of theabsorbance is inputted from the wavelength calibrator and which isconnected to the absorption spectrum data storage, and configured toseparate the absorption spectrum of the absorbance of each of the gasesto be measured from the absorption spectrum of the absorbance;

an area to concentration ratio data storage configure to storewavelength range data used for calculating areas of the absorption linesof the gases to be measured and to store proportional constant data ofthe area and gas partial pressure; and

a concentration detector into which the absorption spectrum of each ofthe gases to be measured is inputted from the spectrum separator andwhich is connected to the area to concentration ratio data storage,configured to obtain an area of a designated wavelength range, and tocalculate partial pressures of the gases to be measured.

In the laser gas analyzer, the wavelength calibrator may compare anabsorption line of a wavelength calibrating gas and known absorptionline.

In the laser gas analyzer, the wavelength calibrator may compare theabsorption line of the wavelength calibrating gas and known absorptionline using polynomial approximation.

In the laser gas analyzer, an absorption rate of a second gas cell inwhich a wavelength calibrating gas is sealed may be smaller than anabsorption rate of the first gas cell.

According to the present invention, it is possible to measure relativelyeasily the concentrations of the respective components included in themulticomponent mixed gas using the area method that does not depend onthe statistical method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a laser gas analyzer according to anexemplary embodiment of the invention.

FIG. 2 is a block diagram showing a specific example of the laser gasanalyzer according to the exemplary embodiment of the invention.

FIG. 3 is a flowchart explaining the entire flow of a measurementoperation of the analyzer as shown in FIG. 2.

FIGS. 4A and 4B are spectrum diagrams of methane.

FIG. 5 is a spectrum diagram of methane, ethylene, ethane, propylene,and propane in the vicinity of 1674.5 nm of Peak P1 in FIG. 4A.

FIGS. 6A and 6B are spectrum diagrams of ethylene.

FIG. 7 is a spectrum diagram of methane, ethylene, ethane, propylene,and propane in the vicinity of 1675.9 nm of Peak P1 in FIG. 6A.

FIGS. 8A and 8B are spectrum diagrams of ethane.

FIG. 9 is a spectrum diagram of methane, ethylene, ethane, propylene,and propane in the vicinity of 1683.1 nm of Peak P2 in FIG. 8A.

FIGS. 10A and 10B are spectrum diagrams of propylene.

FIG. 11 is a spectrum diagram of methane, ethylene, ethane, propylene,and propane in the vicinity of the peak of propylene in FIG. 10B.

FIGS. 12A and 12B are spectrum diagrams of propane.

FIG. 13 is a spectrum diagram of methane, ethylene, ethane, propylene,and propane in the vicinity of the peak of propane in FIG. 12A.

FIG. 14 is a view of an example in which the peak width changes due tothe difference of component gases.

FIG. 15 is a flowchart of a sequence for selecting a proper spectrumfrom the database.

FIGS. 16A to 16C are spectrum diagrams of ethane which is extracted.

FIGS. 17A and 17B are diagrams for explaining a case in which thespectrum area region of ethylene is determined

FIG. 18 is an area calculation region view.

FIG. 19 is a block diagram showing a laser gas analyzer of the relatedart.

DETAILED DESCRIPTION

Hereinafter, embodiments of the invention will be described in detailusing the accompanying drawings. FIG. 1 is a block diagram showing alaser gas analyzer according to an exemplary embodiment of theinvention. In FIG. 1, a wavelength-variable laser 101 generates ameasurement light of absorption spectrums of gases to be measured, andis connected to an oscillation wavelength control circuit 102 thatcontrols the oscillation wavelength.

The emitted light of the wavelength-variable laser 101 becomes aparallel light at a lens 103, passes through an isolator 104, and issplit into two parallel lights of a measurement light and a referencelight using a beam splitter 105.

One of the two parallel lights split using the beam splitter 105 is madeto be incident to a gas cell 106 into which the gases to be measured areintroduced as the measurement light, collected using a lens 107, made tobe incident to a photodiode 108 so as to be converted into electricalsignals, and inputted into one input terminal of wavelength calibrator111.

The other parallel light is collected at a lens 109, made to be incidentto a photodiode 110 as the reference light, converted into electricalsignals, and inputted into the other input terminal of the wavelengthcalibrator 111.

An absorption line wavelength data storage 112 that stores theabsorption line wavelength data of the gases to be measured is connectedto the wavelength calibrator 111. The wavelength calibrator 111calibrates the wavelengths of the spectrum data obtained from thephotodiodes 108 and 110 using the absorption line wavelength data storedin the absorption line wavelength data storage 112 and the absorptionspectrum obtained from the photodiode 108.

Furthermore, the wavelength calibrator 111 performs division of thewavelength-calibrated spectrum data of the photodiodes 108 and 110 so asto compute the absorbance and obtain the absorption spectrum of theabsorbance, and inputs the absorption spectrum of the obtainedabsorbance into spectrum separator 113.

An absorption spectrum data storage 114 that stores the absorptionspectrum data of the gases to be measured is connected to the spectrumseparator 113. The spectrum separator 113 separates the absorptionspectrums of the absorbance of the gases to be measured from theabsorption spectrum of the absorbance inputted from the wavelengthcalibrator 111 using, for example, least squares fitting in which theabsorption spectrums of the gases to be measured are used, and inputsthe obtained absorption spectrums of the gases to be measured intoconcentration detector 115.

An area to concentration ratio data storage 116 is connected to theconcentration detector 115. The area to concentration ratio data storage116 stores wavelength range data that calculate the areas of theabsorption lines of the respective gases to be measured, and stores theproportional constant data of the area and the gas partial pressure. Theconcentration detector 115 obtains the area of a designated wavelengthrange from the absorption spectrums of the absorbance of the respectivegases to be measured inputted from the spectrum separator 113, and,then, computes the partial pressures of the gases to be measured bymultiplying the obtained area by a designated proportional constant. Adata processor 200 of the laser gas analyzer shown in FIG. 1 includesthe wavelength calibrator 111, the absorption line wavelength datastorage 112, the spectrum separator 113, the absorption spectrum datastorage 114, the concentration detector 115 and thearea-to-concentration ratio data storage 116.

FIG. 2 is a block diagram showing a specific example of the laser gasanalyzer according to the embodiment of the invention which measuresmultiple hydrocarbon component, and the same signs are given to portionsthat are common in FIG. 1. The wavelength-variable laser 101 includes afirst MEMS-vertical cavity surface emitting laser (MEMS-VCSEL) thatemits a wavelength of 1620 nm to 1640 nm at which the measurement lightof the absorption spectrum of hydrocarbon is generated and a secondMEMS-VCSEL that emits a wavelength of 1670 nm to 1700 nm. The respectiveMEMS-VCSELs are connected to the oscillation wavelength control circuit102, the oscillation wavelength control circuit 102 alternately appliesa current to the respective MEMS-VCSELs so as to alternately oscillatethe respective MEMS-VCSELs, and controls a voltage applied to theoscillating MEMS-VCSELs so as to control the respective oscillatingwavelengths.

The emitted light of the respective MEMS-VCSEL becomes a parallel lightat the lens 103, passes through the isolator 104, furthermore, passesthrough a wavelength calibrating gas cell 117 having CH₄ and C₂H₄ sealedtherein, and is split into two parallel lights of a measurement lightand a reference light using the beam splitter 105.

One of the two parallel lights split using the beam splitter 105 is madeto be incident to the gas cell 106 into which the gases to be measuredare introduced as the measurement light, and becomes the absorptionsignals of the gases to be measured. The output light of the gas cell106 is collected using the lens 107, made to be incident to thephotodiode 108 so as to be converted into electrical signals, andinputted into one input terminal of the wavelength calibrator 111.

The other parallel light is collected at the lens 109, made to bedirectly incident to the photodiode 110 as the reference light,converted into electrical signals, and inputted into the other inputterminal of the wavelength calibrator 111. Thereby, the referencesignals of the output light intensity of the respective MEMS-VCSELs areformed.

The gas cell 106 is controlled to a predetermined temperature through agas cell temperature controller 118, and the gases to be measured arecontrolled to the same temperature as the gas cell 106 through a gastemperature controller 119.

Among the gases to be measured, the absorption line wavelength data ofCH₄ and C₂H₄ that is sealed as wavelength calibrating gas in thewavelength calibrating gas cell 117 are stored in the absorption linewavelength data storage 112 connected to the wavelength calibrator 111.The wavelength calibrator 111 calibrate the wavelengths of the spectrumdata obtained from the photodiodes 108 and 110 using the absorption linewavelength data of CH₄ and C₂H₄ stored in the absorption line wavelengthdata storage 112 and the absorption spectrums of CH₄ and C₂H₄ obtainedfrom the photodiode 110.

Furthermore, the wavelength calibrator 111 performs division of thewavelength-calibrated spectrum data of the photodiodes 108 and 110 so asto compute the absorbance and obtain the absorption spectrum of theabsorbance of the gases to be measured, and inputs the obtainedabsorption spectrums of the absorbance of the gases to be measured intothe spectrum separator 113.

The absorption spectrum data storage 114 connected to the spectrumseparator 113 stores the absorption spectrum data of hydrocarboncomponents (CH₄, C₂H₄, C₂H₆, C₃H₆, and C₃H₈) which are the gases to bemeasured. The spectrum separator 113 separates the absorption spectrumsof the absorbance of the respective hydrocarbon components (CH₄, C₂H₄,C₂H₆, C₃H₆, and C₃H₈) from the absorption spectrum of the absorbanceinputted from the wavelength calibrator 111 using least squares fittingin which the absorption spectrums of the gases to be measured are used,and inputs the obtained absorption spectrums of the respective gases tobe measured into the concentration detector 115.

The area to concentration ratio data storage 116 connected to theconcentration detector 115 stores wavelength range data that calculatethe areas of the absorption lines of the hydrocarbon components (CH₄,C₂H₄, C₂H₆, C₃H₆, and C₃H₈) which are the respective gases to bemeasured and stores the proportional constant data of the area and thegas partial pressure which has been actually measured in advance.

The concentration detector 115 obtains the area of a designatedwavelength range from the absorption spectrums of the absorbance of therespective gases to be measured inputted from the spectrum separator113, and obtains the partial pressures of the respective gases to bemeasured by multiplying the obtained area by a designated proportionalconstant. Furthermore, the concentrations of the respective gases to bemeasured can be obtained by dividing the obtained partial pressures ofthe respective gases to be measured by the total pressure measured usinga pressure meter 120 provided in the outlet path of the gas cell 106.

FIG. 3 is a flowchart explaining the entire flow of a measurementoperation of the analyzer as shown in FIG. 2. The processing of signalsstarts wavelength calibration using the reference signals that are notinfluenced by the gases to be measured.

The wavelength calibrator 111 detects the data numbers of the peaks ofthe sharp absorption spectrums of CH₄ and C₂H₄ based on the referencesignals outputted from the photodiode 110, and determines the accuratewavelengths of the data numbers from the table of absorption lines andwavelengths. From the relationship between the obtained plural datanumbers and the wavelengths, the respective data numbers, for example, awavelength 1685 nm range, and a 1630 nm range are matched usingpolynomial approximation (Steps S1 to S3).

After wavelength calibration, absorbance is calculated from thereference signals and the absorption signals in the respectivewavelength bands (Step S4), an optimal pure spectrum is selected (StepS5), a rough gas concentration analysis is performed (Step S6), and thenspectrum separation and concentration analyses are repeated sequentially(C₂H₄→CH₄→C₂H₆→C₃H₈→C₃H₆) from components having a larger influence(Steps S7 to S16).

When the incident intensity to the gas to be measured is represented byI₁, the light intensity transmitted through the gas to be measured isrepresented by I₂, and the transmissivity of the gas to be measured isrepresented by A, the absorbance of the gas at a wavelength λ becomes

$\begin{matrix}{\begin{matrix}{{Absorbance} = {\log_{10}\left\lbrack {{I_{1}(\lambda)}/{I_{2}(\lambda)}} \right\rbrack}} \\{= {\log_{10}\left\lbrack {{{I_{1}(\lambda)}/{I_{1}(\lambda)}} \times {A(\lambda)}} \right\rbrack}} \\{= {\log_{10}\left\lbrack {1/{A(\lambda)}} \right\rbrack}}\end{matrix}\quad} & (1)\end{matrix}$

Furthermore, when the light intensity of the reference light isrepresented by I_(r), the light intensity immediately before thewavelength calibrating gas cell 117 is represented by I₀, thetransmissivity of the wavelength calibrating gas is represented by A₁,the transmissivity of the wavelength calibrating gas cell 117 isrepresented by T_(r1), the split ratio by the beam splitter 105 isrepresented by R₁:R₂ (R₁+R₂=1), and the transmissivity of the gas cell106 is represented by T_(r), it is possible to express

λ_(r)(λ)=I ₀(λ)×A ₁(λ)×T _(r1) ×R ₁(λ)  (2)

I ₂(λ)=I ₀(λ)×A ₁(λ)×T _(r1) ×R ₂(λ)×A(λ)×T _(r)(λ)  (3)

Therefore,

$\begin{matrix}{\begin{matrix}{{\log_{10}\left\lbrack {{I_{r}(\lambda)}/{I_{2}(\lambda)}} \right\rbrack} = {\log_{10}\left\lbrack {{R_{1}(\lambda)}/\left( {{R_{2}(\lambda)} \times {A(\lambda)} \times {T_{r}(\lambda)}} \right)} \right\rbrack}} \\{= {{\log_{10}\left\lbrack {1/{A(\lambda)}} \right\rbrack} +}} \\{{\log_{10}\left\lbrack {{R_{1}(\lambda)}/\left( {{R_{2}(\lambda)} \times {T_{r}(\lambda)}} \right)} \right\rbrack}} \\{= {{absorbance} + {{analyzer}\mspace{14mu} {function}}}}\end{matrix}\quad} & (4)\end{matrix}$

Herein, since the second element does not depend on the gas to bemeasured, when a spectrum is obtained in advance by sealing a gas forwhich absorption can be ignored in the gas cell, the second element canbe removed. That is, when the common logarithm is calculated byobtaining the ratio of the output signals of the photodiodes 108 and 110like the first element in the formula (4), the absorbance is obtained.

Meanwhile, attention should be paid to that there are cases in whichboth of I_(r) and I₂ include the influence of the wavelength calibratinggas cell 117. Due to the above fact, like the first element in theformula (4), when the ratio of both signals is obtained, the signals ofthe wavelength calibrating gas cell 117 disappear, and thereforecalculation can be performed easily.

Next, the wavelength bands used when the concentrations (gas partialpressures) of the respective hydrocarbons are detected will bedescribed. The point of the invention is that it is possible to separatethe spectrum of each of the respective gases to be measured(hydrocarbons) from the measured absorption spectrum of the mixed gas,and obtain the concentrations (gas partial pressures) from the area ofeach of the obtained absorption lines.

In order to precisely separate spectrums, wavelength bands in which thecharacteristic absorption lines of the gases to be measured are present,but the characteristic absorption lines of other mixed gas are notpresent are preferably selected. When such wavelength bands areselected, since the major influence of the other gases becomes the sameas that of a change in the base line, the influence of the other gasescan be minimized by employing a concentration (gas partial pressure)detecting method which is not influenced by a change in the base line.Therefore, in the invention, the concentrations (gas partial pressures)are detected using different wavelength bands satisfying the aboveconditions for each of the gases to be measured (hydrocarbons) and amethod which is not influenced by a change in the base line.Hereinafter, the above will be described respectively.

FIGS. 4A and 4B show the spectrum diagrams of methane, in which FIG. 4Aindicates the 1685 nm band, and FIG. 4B indicates the 1630 nm band. Asis evident from FIG. 4, methane has plural sharp peaks in each of thewavelength bands. Among the sharp peaks, sharp peaks having a smallamount of the absorption components of the other gases and that areappropriate for the concentrations (gas partial pressures) detection arePeaks P1 to P6 which are indicated using arrows.

FIG. 5 is a spectrum diagram of methane, ethylene, ethane, propylene,and propane in the vicinity of 1674.5 nm of Peak P1 in FIG. 4. Accordingto FIG. 5, it is found that the peak of methane is evidently sharp andlarge compared to those of other gas spectrums. Thereby, the influenceof other gases can be considered to be a change in the base line.

FIGS. 6A and 6B show the spectrum diagrams of ethylene, in which FIG. 6Aindicates the 1685 nm band, and FIG. 6B indicates the 1630 nm band. Asis evident from FIG. 6, ethylene also has plural sharp peaks in each ofthe wavelength bands; however, in the 1630 nm band, since the peakintervals are narrow, it is difficult to extract specific peaks. Incontrast to the above, Peaks P1 to P3 indicated using arrows in the 1685nm band are appropriate for concentration (gas partial pressure)detection.

FIG. 7 is a spectrum diagram of methane, ethylene, ethane, propylene,and propane in the vicinity of 1675.9 nm of Peak P1 in FIG. 6. Accordingto FIG. 7, it is found that the peak of ethylene is evidently sharp andlarge compared to those of other gas spectrums. Thereby, among the sharppeaks, Peak P1 can be said to have a small amount of the absorptioncomponents of other gases and be appropriate for concentration (gaspartial pressure) detection.

FIGS. 8A and 8B show the spectrum diagrams of ethane, in which FIG. 8Aindicates the 1685 nm band, and FIG. 8B indicates the 1630 nm band. Asis evident from FIG. 8, ethane has no peak in the 1630 nm band, and hasseveral peaks in the 1685 nm band. Among the peaks, Peaks P1 to P3indicated using arrows are appropriate for concentration (gas partialpressure) detection.

FIG. 9 is a spectrum diagram of methane, ethylene, ethane, propylene,and propane in the vicinity of 1683.1 nm of Peak P2 in FIG. 8. Accordingto FIG. 9, ethylene and methane have absorption in the vicinity of1683.1 nm, and the absorption of ethane is highest.

FIGS. 10A and 10B show the spectrum diagrams of propylene, in which FIG.10A indicates the 1685 nm band, and FIG. 10B indicates the 1630 nm band.As is evident from FIG. 10, propylene has only one peak at 1628.7 nm,and this peak is used for concentration (gas partial pressure)detection.

FIG. 11 is a spectrum diagram of methane, ethylene, ethane, propylene,and propane in the vicinity of the peak of propylene in FIG. 10.According to FIG. 11, since absorptions of ethylene and methane arelarge, a method in which propylene is extracted by removing theinfluence of these gases becomes important.

FIGS. 12A and 12B show the spectrum diagrams of propane, in which FIG.12A indicates the 1685 nm band, and FIG. 12B indicates the 1630 nm band.As is evident from FIG. 12, propane has only one peak at 1686.4 nm, andthis peak is used for concentration (gas partial pressure) detection.

FIG. 13 is a spectrum diagram of methane, ethylene, ethane, propylene,and propane in the vicinity of the peak of propane in FIG. 12. Accordingto FIG. 13, since absorptions of ethylene and methane are large, amethod in which propane is extracted by removing the influence of thesegases becomes important.

Next, a spectrum separation method will be described. Here, attentionshould be paid to that the shape of the absorption spectrum changes dueto total pressure, gas concentrations (gas partial pressures), and thekinds of the mixed gas. When the changes are not dealt with, precisespectrum separation is not possible.

Therefore, in the invention, the absorption spectrums of the respectivegases to be measured in consideration of a change in the spectrum shapeare used for spectrum separation.

A specific example in which the peak width changes will be describedusing FIG. 14. FIG. 14 is a view of an example in which the peak widthchanges due to the difference of component gases, only the absorptionspectrum of methane is extracted by removing absorptions of other thanmethane. In FIG. 14, Characteristic A indicates the spectrum of a mixedgas of 50% of methane and 50% of hydrogen, and Characteristic Bindicates the spectrum of a mixed gas of 50% of methane and 50% ofpropylene. As is evident from FIG. 14, the peak width becomes wider whenmethane is mixed with propylene compared to when methane is mixed withhydrogen.

In order to deal with the change in the spectrum shape, the absorptionspectrums of the respective gases to be measured which are most similarto the actual spectrum shape are selected from a spectrum databaseobtained in advance using a processing sequence method shown in theflowchart of FIG. 15. In order to select an absorption spectrum which issimilar to the spectrum shape, the spectrum database should include theabsorption spectrums of a variety of shapes.

The cause for a change in the spectrum shape is considered to befrequent occurrence of molecular collision. That is, propylene orpropane having a large collision cross-sectional area of molecules has alarge effect of changing the spectrum shape. On the other hand, examplesof molecules having a small effect of changing the spectrum shapeinclude a nitrogen molecule having a small collision cross-sectionalarea of molecules.

Based on the above reason, a spectrum database is built by measuring anumber of spectrums for which the mixing ratio of a combination of twokinds of components in the mixed gas shown below is changed, and aspectrum having the most similar spectrum shape is selected and usedfrom the database.

-   -   CH₄:CH₄+N₂, CH₄+C₃H₆, CH₄+C₃H₈    -   C₂H₄:C₂H₆+N₂, C₂H₄+C₃H₆, C₂H₄+C₃H₈    -   C₂H₆:C₂H₆+N₂, C₂H₆+C₃H₆    -   C₂H₆:C₂H₆+N₂    -   C₃H₈:C₃H₈+N₂

Based on the flowchart of FIG. 15, a sequence for selecting a properspectrum from the database will be described. Firstly, the absorptionspectrum of each of the respective gases to be measured which is to beused in calculation (hereinafter the initial pure spectrum) is read fromthe database. Herein, it is not necessary to consider the change in thespectrum shape and the like, and an arbitrary pure spectrum may be used(Step S1).

Next, a gas selection loop begins (Step S2), and a kind of gas thatdetermines the optimal pure spectrum is selected (Step S3). In addition,from a mixed gas spectrum, a wavelength area portion in which theselected gas has characteristic peaks and which is selected in advanceis cut out (Step S4).

Subsequently, a pure spectrum determination loop begins (Step S5). Withregard to the selected gas, pure spectrums are selected from thedatabase one by one, and, with regard to the spectrum of other than theselected gas, the pure spectrum of Step S1 is selected (Step S6).

Using the pure spectrum, a spectral residual is obtained in thefollowing formula manner.

A=CK+R  (5)

Here, A represents the mixed gas spectrum, K represents the arrayed purespectrums of the respective gases, C represents the concentrations (gaspartial pressures) of the respective gases, and R represents thespectral residual. Here, when the pure spectrums of the respective gasesare determined, the concentrations (gas partial pressures) can beobtained from the formula (5) in the following formula (6) manner.

C=AK ^(T)(KK ^(T))⁻¹  (6)

The concentrations (gas partial pressures) obtained in the above mannerare again substituted into the formula (5) so as to perform the squaresum computation of the spectral residual (Step S7).

After the square sum of the spectral residual is recorded (Step S8), ananother pure spectrum is selected for the selected gas, and the squaresum of the spectral residual is recorded. The above step is performed onall the pure spectrums of the selected gas (Step S9). Among the squaresums of the spectral residual obtained in the above manner, the purespectrum when the square sum is least is used as the optimal purespectrum (Step S10).

After the optimal pure spectrum is obtained, the same flow is repeatedfor the next gas, and the optimal pure spectrums are obtained for allthe gases (Step S11).

In Step S6 of FIG. 3, the formula (6) is recalculated in the vicinity ofthe peaks of the respective gases using the obtained optimal purespectrums, and the approximately calculated concentrations (gas partialpressures) of the respective gases are obtained. Only the peak of acertain hydrocarbon can be extracted by subtracting the optimal purespectrum of other gases for the concentration (gas partial pressure)obtained using the formula (6) from the spectrum in the vicinity of thepeak selected for the hydrocarbon. For example, in a case in which onlyethanol is extracted from the spectrum of a mixed gas of 5 kinds ofhydrocarbons (methane, ethylene, ethane, propylene, and propane), it isallowed to be that

A(C₂H₆)=A−K(CH₄)C(CH₄)−K(C₂H₄)C(C₂H₄)−K(C₃H₆)C(C₃H₆)−K(C₃H₈)C(C₃H₈)  (7)

When values obtained using the formula (6) in the vicinity of therespective peaks are used as the concentrations (gas partial pressures)of the respective hydrocarbon gases being used in the formula (7), theextraction accuracy increases. The spectrum of ethane which is extractedin the above manner is shown in FIGS. 16A to 16C. FIG. 16A shows thespectrum of the mixed gas, FIG. 16B shows the spectrum of extractedethane, and FIG. 16C shows the pure spectrum of ethane. As is evidentfrom FIGS. 16A to 16C, while ethane is included only at 15%, a spectrumof ethane having an almost the same shape as the pure spectrum can beextracted.

Meanwhile, in a case in which the influence of a change in the base lineremains, the change in the base line can be removed by performing leastsquares fitting using zero-order (simple offset), first-order (slantoffset), and the pure spectrum of ethane. In the invention, correctionis made up to the first element so that the absorption spectrums of therespective gases to be measured from which the change in the base lineis removed are separated.

In order to calculate the concentrations (gas partial pressures) usingthe extracted spectrums, the areas of the spectrums are used. When thepartial pressures of the respective gases are constant, a region inwhich the proportional relationship between area and concentration (gaspartial pressure) is formed at all times is present even when externalenvironments such as the total pressure and component gases change. Thepartial pressures of the respective gases are calculated using the areaof the region.

As a specific example, a case in which the spectrum area region ofethylene is determined will be described using FIGS. 17A and 17B.Firstly, in order to subtract the base line portion, for C₂H₄, astraight portion connecting two points of the X point on the shortwavelength side and the Y point on the long wavelength side of the peakportion is subtracted as shown in FIG. 17A. Thereby, the influence ofthe change in the base line can be completely removed as shown in FIG.17B. In addition, the area between the two points is obtained.

The above processing is performed in the same manner on all thespectrums in the spectrum database for which the concentrations (gaspartial pressure) are already known. FIG. 18 is an area calculationregion view which shows the result of a combination of X and Y obtainedwhen the relationship between concentration (gas partial pressure) andarea is most similar to a proportional relationship by changing the Xand Y values in a range of 1 to 400. The horizontal axis corresponds toX, the vertical axis corresponds to Y, the blue portion (B) indicatesthat the detection error decreases, and the red portion (R) indicatesthat the detection error increases. For ethylene, a combination of X=317and Y=373 becomes effective.

A gas chromatography has been mainly used for analyses of hydrocarbonmulticomponent systems due to the high component separation capability;however, since the measurement time was long, direct use of measuredvalues for control was not possible. A laser gas analyzer which canmeasure on a real time basis has a narrow wavelength-variable width of alight source, and is thus limited mainly to measurement of a singlecomponent.

In contrast to the above, in the invention, an industrial laser gasanalyzer which can measure the absorption spectrum of a multicomponentmixed gas on a real time basis was realized by using a MEMS-VCSEL havingno mechanical moving portion, high reliability which is required as anindustrial instrument, and a capability of wavelength variation in awide wavelength range as a light source.

Specifically, a laser gas analyzer which can measure the absorptionspectrum of a multicomponent mixed gas on a real time basis and cananalyze a hydrocarbon multicomponent mixed gas was realized by usingwhen a MEMS-VCSEL oscillating in a wavelength range of 1620 nm to 1750nm as a light source.

A precise absorption spectrum can be obtained in a wide wavelength rangeby performing wavelength calibration using the absorption lines of CH₄and C₂H₄.

A case in which it becomes impossible to measure absorption lines thatare used for wavelength calibration due to a decrease in theconcentrations (gas partial pressures) of gases being used forwavelength calibration among the gases to be measured can be avoided byusing the wavelength calibrating gas cell 117 in which CH₄ and C₂H₄ aresealed.

In an analyzer of the related art, the wavelength calibrating gas cell117 is inserted into a path for the reference light separated at thebeam splitter 105; however, in the invention, the wavelength calibratinggas cell 117 is placed ahead of the beam splitter 105 so that wavelengthcalibration and light output calibration can be performed at onelight-receiving element. In addition, the absorption signals of acalibrating gas can be subtracted from the absorption signals of thegases to be measured by simply performing division of the absorptionsignals of the gases to be measured obtained from the photodiode 108 andthe reference signals obtained from the photodiode 110.

An accurate absorption spectrum is obtained by performing the wavelengthcalibration of the absorption spectrum of the hydrocarbon multicomponentmixed gas obtained using the absorption lines of CH₄ and C₂H₄ of thewavelength calibrating gas and the wavelength table.

The absorption spectrum of each of the respective gases is obtained byseparating the absorption spectrums of the respective components usingthe absorption spectrum database of the respective components in thehydrocarbon multicomponent mixed gas.

It become unnecessary to obtain a calibration curve for each applicationand excessive engineering man-hours can be decreased by developing andemploying the area method in which the statistical method (chemometrics)is not used as a module for obtaining the concentrations (gas partialpressures) of the respective components from the absorption spectrum ofa multicomponent system. Thereby, the absorption spectrum of each of therespective gases can be obtained, and the concentrations (gas partialpressures) of the respective gases can be obtained without using thestatistical method (chemometrics).

In order to use the area method, it is necessary to separate theabsorption spectrum of each of the respective gases to be measured fromthe overlapped absorption spectrum of the mixed gas; however, in theinvention, least squares fitting was performed in a wavelength band inwhich the characteristic absorption lines of the respective gases arepresent using the absorption spectrum database including even the changein the absorption spectrum shape due to mixing of gases, and theabsorption spectrum of each of the respective gases to be measured wereseparated from the absorption spectrum of the mixed gas. Thereby, theinfluence of the absorption of other gases can be suppressed to anextent of the change in the base line.

In the invention, since a method in which an area is obtained bysubtracting a value determined from the shape of absorption lines fromthe absorption spectrum is employed as the area method, the influence ofthe change in the base line can be removed.

It became unnecessary to prepare calibration curves by using wavelengthregions in which absorption of other hydrocarbon gases was small andabsorption of impurities was small as the peaks of the respectivehydrocarbon gases.

A change in the spectrum shape could be dealt with by selecting the mostappropriate spectrum for the actual spectrum shape from the spectrumdatabase, whereby robustness improved.

It became possible to highly accurately measure the concentrations (gaspartial pressures) by using the area of a region in which theconcentration (gas partial pressure) error decreases even when thespectrum shape changed.

Meanwhile, depending on the concentration (gas partial pressure) value,there is a possibility that other gas spectrums increase in the vicinityof the selected peaks such that the extraction error increases. In sucha case, peaks being used may be changed depending on the concentration(gas partial pressure) values of the mixed gas.

The temperature of the gases to be measured inputted to the gas cell 106is desirably set to be constant using the gas temperature controller119. Thereby, a change in the spectrum shape accompanying a change inthe temperature can be suppressed, and the measurement error due to achange in the temperature can be reduced.

When the optical system is configured using an optical fiber,restrictive conditions in designing can be mitigated so as to achieveminiaturization, and stability with respect to mechanical vibrations canbe increased.

Application to gases that are not easily absorbed or detection of asmall amount of gas becomes possible by providing multiple paths so asto extend the light path length.

The gases to be measured may be depressurized and measured. Thereby,separation of overlapped absorption lines can be expected.

Time-division measurement is also possible at a common light-receivingelement by using plural wavelength-variable light sources.

The wavelength calibrating gas is not limited to CH₄ and C₂H₄, and C₂H₄or HCl may also be used depending on the wavelength band which is usedfor measurement. Examples of the wavelength calibrating gas being usedinclude gases of molecules having a small number of atoms and sharpabsorption lines, gases which are the same gases to be measured,isotopes of the gases to be measured, or gases having a similarcomposition which has absorption lines in a close wavelength range. Forexample, when the spectrum of the hydrocarbon is measured, wavelengthcalibration is performed using a hydrocarbon having one or two carbonatoms.

The total pressure of the wavelength calibrating gas is desirablydepressurized in order to sharpen the absorption lines being used forcalibration. Particularly, more highly accurate measurement becomespossible through depressurization so that the overlapping of pluralabsorption lines is solved and separated. For example, when methane isused for wavelength calibration, it is effective to decrease the totalpressure to 0.1 atmospheres or less since the overlapping of pluralabsorption lines is solved.

The absorption rate of the wavelength calibrating gas cell 117 isdesirably set to 1/10 or less of the absorption rate of the gas cell 106in order to decrease the influence on the S/N of measurement signals.

As described above, according to the invention, a laser gas analyzer inwhich a wavelength-variable laser having a wide wavelength-variablewidth as a laser light source is used, and the concentrations of therespective components included in the hydrocarbon multicomponent mixedgas can be measured relatively easily using the area method that doesnot depend on the statistical method can be realized, which is effectivefor direct measurement of a variety of process gases.

What is claimed is:
 1. A laser gas analyzer configured to analyze gasesto be measured in a multicomponent mixed gas by using a tunable diodelaser absorption spectroscopy method, comprising: a wavelengthcalibrating gas cell that calibrates wavelengths of an absorptionspectrum, wherein a wavelength calibrating gas, which includes two ormore gas components, is sealed in the wavelength calibrating gas cell.2. The laser gas analyzer according to claim 1, wherein the wavelengthcalibrating gas includes gas which is the same kind as the gas to bemeasured.
 3. The laser gas analyzer according to claim 1, wherein thewavelength calibrating gas includes an isotope of the gas to bemeasured.
 4. The laser gas analyzer according to claim 1, wherein thewavelength calibrating gas includes CH₄ and C₂H₄.
 5. The laser gasanalyzer according to claim 1, wherein the wavelength calibrating gas isdepressurized.
 6. The laser gas analyzer according to claim 2, whereinthe wavelength calibrating gas is depressurized.
 7. The laser gasanalyzer according to claim 3, wherein the wavelength calibrating gas isdepressurized.
 8. The laser gas analyzer according to claim 4, whereinthe wavelength calibrating gas is depressurized.
 9. A laser gasanalyzer, comprising: a wavelength-variable laser having awavelength-variable width to cover a range of wavelengths; a light-splitmodule configured to split an output light of the wavelength-variablelaser into a measurement light and a reference light; a measurement gascell into which the multicomponent mixed gas is introduced, and intowhich the measurement light is made to be incident; a data processorconfigured to obtain an absorption spectrum of each component of thegases to be measured included in the multicomponent mixed gas based on areference signal related to the reference light and an absorption signalrelated to an output light of the measurement gas cell, and to obtainconcentrations of the respective components of the gases to be measured;and a wavelength calibrating gas cell that calibrates wavelengths of anabsorption spectrum, in which a wavelength calibrating gas including twoor more gas components is sealed, wherein the wavelength calibrating gascell is placed in front of the light-split module.